Assessment of Tropospheric Anomalies for CAT II/III GBAS Based on IGS Products

Ground-based augmentation systems (GBASs) broadcast differential corrections and integrity information to airborne users based on very high frequency (VHF) data broadcasting stations, where the tropospheric delay is one of the most important corrections required. In GBAS, it is assumed that the errors caused by signal propagation is approximately equal for the reference station and the user and remain stable over a short time. However, this assumption is not always satisfied in abnormal atmospheric situations. In this study, 13 years International GNSS Service (IGS) zenith tropospheric delay (ZTD) products are first used to analyze temporal and spatial anomalies in the troposphere. It is concluded that tropospheric time gradients identified using inter-epoch difference method are rare, whereas spatial gradients identified using a station-pairing method cannot be neglected. The tropospheric delay difference between the CIT1 (34.137° N, 118.127° W) and WLSN (34.226° N, 118.056° W) stations, which are located on the west coast of the United States, is found to exceed 200 mm on the 47^th Day in 2020. To ensure the authenticity of anomaly gradients, cross verification is performed using precision point positioning (PPP) technology based on real GNSS data. A theoretical derivation based on existing scheme is carried out to resolve the nonuniqueness of the atmospheric scale height obtained using the existing calculation scheme. The results show that the atmospheric “scale height” has no physical meaning but is only a fitting coefficient. The existing calculation scheme for calculating tropospheric integrity parameters is optimized. Modifications to the existing calculation scheme are recommended. Finally, as an example, tropospheric integrity parameters are calculated using the optimized algorithm and meteorological data from the Dongying Shengli Airport in Shandong Province, China from 2018 to 2019. The tropospheric integrity parameters of the airport are found to be N_R = 334.5 and σ_N = 28.24.